Echo suppressor



Maxl'ch 20, 1934. s. B. WRIGHT ET AL ECHO SUPPRESSOR Filed Aug. 19, 19302 Sheets-Sheet l 'lill'.i.-

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l INVENToRs f. @f/ Mdc/ed BY L ATTORNEY March 20, 1934- s. B. WRIGHT ErAL ECHO SUPPRESSOR Filed Aug. 19, 1930 2 sheets-sheet 2 .EO 5.3% Q ENwww:

. L' lll INVENTORS 5.29. mgmmfa'/wa ATTORNEY Patented Mar. 20, 1934UNITED STATES ECHO SUPPRESSOR Sumner B. Wright, South Orange, N. J., andDoren Mitchell, New York, N. Y., assignors to American Telephone andTelegraph Company, a corporation of New York Application August 19,1930, Serial No. 476,422

4 Claims.

This invention relates to two-way transmission circuits, and moreparticularly to four-wire circuits and the arrangements often associatedtherewith for suppressing echoes.

5 For a discussion of some of the fundamental problems connected withthe presence of echoes in two-way transmission circuits and for adescription of echo-suppressing devices which are among those betterknown in the art, reference may be had to an article by A. B. Clark andR. C. Mathes, Journal of the American Institute of Electrical Engineers,June 1925, pages 618-626.

1n United States Patent No. 1,826,196, issued to L. G. Abraham October6, 1931, there is disclosed the so-called terminal echo suppressor,which accomplishes generally satisfactory suppression oi echoes in longtransmission circuits without the requirement of a hangover time of suchlength as to be undesirable.

In United States Patents Nos. 1,814,017 and 1,814,018, issued to S. B.Wright and D. Mitchell July 14, 1931, and in United States Patent No.1,772,551 issued to D. Mitchell and H. C. Silent August 12, 1930, thereis disclosed the so-called noise desensitizing echo suppressor, in whichnoise is rendered ineffective to operate the echo suppressor relays andyet the sensitivity of the device to voice currents remains unimpairedwhen no noise is present.

One of the objects of the present invention is to combine in asatisfactory echo suppressor arrangement the advantages of the terminaland the noise desensitizing Suppressors.

A further object of this invention is to provide an arrangement forsuppressing echoes which is at once free from. false operation andexcessive hangover and sufciently reliable to be satisfactory for use ontransmission circuits having long four-wire sections.

These objects are attained by utilizing the terminal echo suppressorprinciple, employing one or more noise desensitizing circuits andemploying a novel arrangement of relays and associated circuits, as willappear clearly from the ."2 description of the apparatus and itsoperation The disclosure of Fig. 1, in addition to showingthe iirstarrangement, helps to bring out the purpose,

advantages and arrangement of the modied system of Fig. 2. Wherever suchdesignation appears helpful to the reader of the diagrams, like refer-vence characters are used in the two figures of the drawings todesignatecorresponding elements.

With reference to the details of the drawings, and first with particularreference to Fig. 1 thereof, there is shown a four-wire circuit havingits upper two-wire path adapted for transmission from west to east, forinstance, and its lower two-wire path adapted for transmission from eastto west. tits west end the four-wire circuit is connected through thehybridV coil HC1 to the two-wire line L1, this line being balanced bythe network N. At theY east end the four-wire circuit is similarlyconnected through the coil HC1 to the two-wire line L2, which isbalanced by the network N. It will be understood that# the echosuppressing apparatus is located at or quite near the west end of thefour-wire circuit in the one case, and at or near the east end of thefourwire circuit in the other case, and'that the circuit rnay be verylong, with no intermediate echo suppressing devices.

In the transmission line L there are disclosed two one-way ampliers A1at the west end and As at the east end; likewise, in the line L thereare found the one-way amplifiers A1 at theV east end and A3 at the westend. It will be noted.

that on the output side of each of these four ampliers there is anarrangement of resistances, which may be quite small, and repeatingcoilsl and that the apparatus associated with thesev four amplifiers isalso associated with the inputs of one-way ampliers A2, A4, Az and A4,respectively.

With reference to the above-identified disclosures of the terminal echosuppressor and the noise desensitizing echo suppressor, it will be seenthat the arrangement of Fig. 1 of the present application combines theadvantages of these two echo suppressing devices. When currents arriveat the west end of the circuit from the east end over' line L', relay R2connected in the output of detector D2 will operate to placev a shortcircuit on the two-way line L unless earlier arriving cur-v rentsflowing in the line L have broken the operating circuit through relay R2through the action of detector D1 and relay R1. Likewise, it will beseen that the desensitizing circuit S functions to prevent the operationof either the line shorting relay R2 or the auxiliary control relay Rrin response to noise currentsr in either side of the four-wire circuit.

At theeast end of the four-wire circuit the echo suppressing apparatusis similar to that more fully disclosed in Fig. 1 as connected at thewest end of the circuit. The detectors D1 and D2 corresponding todetectors D1 and D2 at the opposite end, are shown only schematically,and, likewise, the desensitizing circuit S', schematically shown, willbe understood to correspond to the desensitizing circuit S at theopposite end.

, The most satisfactory understanding of the arrangement of Fig. 1 willbe obtained, it is believed, from the following description of theoperation of the circuits of that gure:

Transmission from line L1 at the west end of the four-Wire circuit toline L2 at the east end, passes through the amplifier A1 and on over thetwo-wire line L. At the output of amplifier A1 a voltage drop is causedin the small resistances connected in the line. This potential isstepped up through the repeating coils, is added to the voltage dropacross the line,and. is applied to the input of amplier A2. It will beunderstood that these transformer arrangements at the output of the lineamplifiers are such that any transmission passing into the lower branchof the four-wire circuit, due to slight unbalance through the hybridcoil HC1 will not affect amplifier A4.

From the output of amplifier A2 the energy passes into the detector D1and into the desensitizing circuit S through the hybrid coil HC2. Thiscoil, however, is so arranged that the output energy of amplifier A2cannot pass through the coilin any appreciable amount to aifect detectorD2. Detector D1 without the intervention of the other elements would, ofcourse, operate in response to either voice or noise energy of sufcientintensity passing through amplifier A2 to actuate R1. The winding W1 ofrelay R1 is included in the output circuit of detector D1, and if. thiswinding is energized without the energization of either one of ,theother windings W2 and W3, the relay will operate to break the outputcircuit of detector D2 through winding W2 of relay R1 and winding W4,the operating winding of relay R2; Accordingly, given voice currentswithout considerable noise energy passing fromy West to east over lineL, the disabling of the echo suppressor at the west end of the circuitis assured, and the talker connected with line L1 hasproper control ofthe transmission circuit. It Will be understood, of course, lthat ifbefore the voice currents from L1 reach lamplifier A1, voice'currentspassing in the opposite direction have reached ampliiier A3, detector D2will have operated to energize winding W4 of relay R2, thus placing ashort circuit on line L, and will have energized the winding W2 of relayR1 to bias this relay against operation by any current entering detectorD1.

VIf the energy passing through amplier A2 from the west end of thecircuit or A4 from the east end of the circuit is sustained noise, itcauses the operation of the desensitized circuit S, described in somedetail in the above-identified patent to Mitchell and Silent. It will besufficient to state here that this circuit comprises the vacuum tubesVI1, VT2, VTs and VT4, the plate lament circuit of the last tube VT4including the biasing windings Ws of relay R1 and W5 of relay R2. Thecondensers C and C1 and the corresponding resistance arrangement r andr1 operate in response to noise to cause an increased current ilow inthe output of tube VT4, and the relays R1 and R2 are biased againstoperation by'detector D1 or detector D2, respectively. It will beunderstood that if the energy is .voice energy as distinguished fromnoise, the desensitizing circuit S operates in a manner to prevent thebuilding up of a bias on the relays R1 and R2.

At the east end of the four-wire circuit the echo suppressor operationis quite similar to that described above in connection with the west endof the circuit. Voice currents arriving over line L and reaching thesuppressor point before the arrival of voice currents from L2, operatedetector D2, energize the operating winding W4 of relay R2 and thebiasing winding W2 of relay R1, and, Without opposition, short-circuitthe line L at the east end of the four-wire circuit. If voice currentsfrom L2 reach the suppressor point first, detector D1 is operated, andthe relay R1 operates to break the circuit through relay R2; thuscontrol of the transmission circuit is assured to the talker connectedwith line L2. Noise reaching the hybrid coil HC2 from either directionoperates the desensitizing circuit S to bias relays R1 and R2 againstfalse operation.

Relay R2 and relay R2 should each have a small hangover time provided,corresponding to the time required for echoes to pass over shortterminal circuits which may be connected to the four-wire circuit andwhich may be unequipped with echo suppressors. It is one of the chiefadvantages of the terminal echo suppressor arrangement, however, thatsuch hangover time isvery short in comparison with that heretoforerequired. v While the arrangement of Fig. 1 described above will in allprobability meet the echo suppressing requirements of the greater numberof circuits, there is the possibility that certain troubles in theoperation of the suppressor apparatus will arise due largely to the factthat the control of the West suppressor, for instance, through theassociatedV desensitizing circuit will be broken when noise is cut offby the operation of the suppressor relay at the east end. Thedesensitizing circuit during this quiet period willffall back to acondition in which no appreciable biasing current ows in the windings ofthe relays. Accordingly, when the short circuit is removed at the farend of the circuit yand noise again comes on, the echo suppressing relayat the West end may be momentarily operated-before the desensitizingcircuit has had a chance to build up again the biasing current in therelays, and the result will be a brief mutilation of speech. With thispossibility of diculty in mind, the applicants disclose in Fig. 2 of thedrawings a modified arrangement which will remove the difficulty;

With particular reference to Fig. 2 of the drawings, at the West end ofthe four-wire lcircuit the transmission lines L1, L and L', the hybridcoil HC1, the balancing network N, the one-way ampliers A1, A2, A3 andA4, and the transformer arrangements at the outputs of A1 .and A3, arein arrangement identical with that shown in Fig. 1 and discussedhereinabove. It will readily be understood that at the east end of thefour- Wire circuit the arrangement of transmission lines and amplifiersand the arrangement of echo suppressor apparatus are similar to thosedisclosed in detail at the West end. Just as in Fig. 1, the two ends ofthe circuit correspond in these respects. Accordingly, the descriptionof the arrangement and operation of the echo suppressor devicesand'circuits will, in general, be conned to the west end of the circuitwithout confusion to the reader.

It will be noted that in the arrangement of Fig. 2 there are twodesensitizing circuits S1 and S2, the former .one being associated withthe amplifiers A2 and A4 through the peculiar hybrid coil device HG2. Inaddition there are employed a greater number of relays, with connectionsand functions to be described hereinbelow. As indicated above, the mainobject of the modification of the circuit oi Fig. 1 is to permit thedesensitizing circuits at the west end of the four-wire circuit, forinstance, to remain at a setting determined by the value of steady noisewhich has been arriving at this west terminal during a period when thetransmission from the east end is cut ofi due to the echo suppressoraction at the east end. In other words, when the noise comes on again,the echo suppressor at the west end is found in the condition in whichit was placed by the noise from the east end at the time of the break inthe transmission, and so there is prevented the false operation of thewest echo suppressor by a renewed transmission of noise. It will benoted that this is the only respect in which the action thedesensitizing circuit, as described above and in the patent to Mitchelland Silent, is changed or interfered with.

in view of the discussion of the circuits of Fig. 1 given above, it isbelieved that the arrangement and action oi the circuits of Fig. 2 willbe readily understood from the following description of their operation:

Transmission outgoing from the west terminal of the four-wire circuitpasses through ainpliiier A1, and energy is drawn off and passed throughamplifier A2 to detector D1 and the upper desensitiz. ig circuit S1, itbeing understood that the hybrid coil HG2 has the function of preventingenergy coming to it from either direction from passing beyond it in anysubstantial amount. Thus, in the case now being considered, the lowerlesensitiaing circuit S2 and the detector D2 are unaffected.

When detector D1 operates, the operating winding W1 o relay R1 isenergized. The energy passing into the desensitiaing circuit S1 has theaction described above in connection with Fig. l;

- that is, if it is voice energy it has no appreciable effect on therelays, while if it is noise energy it operates to bias the relaysagainst actuation. Let it be assumed ior the moment that the energypassing through amplier A2 is largely speech energy. Relay rs1 operatesand closes a circuit through the winding oi a rela7 R5. This relay isgiven a suitable slow release time to prevent it from falling baci:between syllables. "in the rrawings t is shown accomplished by theaction of the condenser C3 and the resistance 1'3. The function or"relay R5 is to remove, and hold removed for the proper period, theground connection of the circuit including the winding of the suppressorrelay proper R4, which, incidentally, is provided with a slow releasetime to take care oi" delayed echo in circuits connected to L1. Theoperation of relay R5, addition to removing ground from relay R4, causesthe discharge of a condenser Cr and the consequent operation of a relayRe. relay Re is given a slow release time which equals 2d, when drepresents the time required for an echo to travel the length of thefour-wire circuit plus the length of the connected two-wire line L2 andany circuits connected thereto. The operation of relay Re removes oneground connection from the operating circuits of relays R7 and R8associated with the desensitizing circuits S1 and S2, respectively. Itwill be noted that, as discussed more fully hereinbelow, a second groundis furnished for these relays R7 and R8 through the armature of relayR3; relay Re in removing the ground throughits armature provides for therelease of relays R7 and R8 during the periods when noise energy fromthe east end ofthe circuit is cut off by the echo suppressor operationat the eastV end, and the lower desensitizing circuit S2 is consequentlynot receiving energy from the line L.

rIhe relays R7 and Rs are arranged so that nornially they are inoperated condition and through their armatures provide for the normalconnection of condensers C1 and C2 with the resistance arrangement r1and T2 in the one case, and the normal connection of condensers C1 andC2 with resistances ri and r2 in the case or" the lower circuit. When,however, rrelays R7 and Rs are in any way released, these condensers aredisconnected from their respective resistances, and, as will beunderstood by those skilled in the art, the voltages on the grids of thetubes VT4 and VTC; will be maintained at t eir last values. Accordingly,while these two relays are released no change of any appreciable amountcan be made in the biasing current furnished by the two desensitizingcircuits to the relays.

It has been stated above that the operation of relay Re removes oneground from the operating circuits of relays R7 and R8, and that theother ground is furnished through the armature oi relay R3. Relay Ra hastwo windings W7 and We. Winding W7 is included in the output circuit oftube VTi ci the lower desensitizing circuit and causes a bias to beapplied to the relay which is normally not quite sufficient to releasethe armature and break the ground contact. Winding We is included in theoutput circuit of the lower detector D2 and is normally suijcientlyenergized to hold relay Re operated. It should be stated at this pointthat, preferably, provision is made for the operation of relay R3 whenno noise whatever is present in the line L; as will be understood bythose skilled in the art, this bias may be provided by suitable voltageon the grid of the detector tube D2, or by any other suitable means.

Let it be assumed now that noise is coming over the line L from the eastend oi the fourwire circuit. This Vnoise energy passes through amplifierA4 and enters both of the desensitizing circuits. Through the operationor" the circuit S1 a bias is built up in the winding W3 of relay Ri andwinding W5 of relay R2. through the operation of the circuit S2 thebiasing action described above it set up in windings W7 of relay R3.These noise currents, of course, affect detector D2 as well as thedesensitizing circuits, and the current in winding We of relay Rs willbe sufficient to hold that relay operated. The current in the otherwinding of the D2 output circuit, W4 of relay R2, however, will not besuiiicient to cause the operation of that relay. When speech energypassing from west to east over line L reaches the east end of thefour-wire circuit, the echo suppressing relay (corresponding to R4) willdisable the line L', and the noise energy passing toward the west end ofthe four-wire circuit will be momentarily cut off. Relay Re, however,has been operated as described above and is held in operated conditionby the hangover action described above; accordingly, the right-handground for the operating circuits of relays R7 and R8 is removed. Whenthe noise in line L ceases, the current from detector D2 will ceaseimmediately. The current from VT4 will not drop down so quickly, and

Likewise,

tent, because the action of detector D2 and relay R3 takes place in atime which is short compared to the time required for the charging ofcondensers C1 and C2.

From the above discussion, and from a consideration of the hangover timeof relay Rs, it will be understood that during a period of time equal tothe longest quiet interval which can occur due to the suppression ofnoise by the speech passing over line L, relays R7 and R8 will be released, and the proper bias to prevent subsequent false operation-of theecho suppressor by noise will be maintained on the relaysVK at the westend of the four-wire circuit.

It should be noted that if noise energy .passes from west to east over'line L, it cannot cause the release of relays R7 and Rs by itself, eventhough relay Re has operated to remove the righthand ground from theoperating circuits oi relays R7 and Rs: Ythis is true because nosubstantial amount of this noise energy can pass the coil HG2 and enterthe lower desensitizing `circuit YS2 to release relay R3 and remove theleft:- hand ground. This point makes it clear why it is desirable to usethe two desensitizing circuits: if the circuit S1 were made to periormthe function of Sz' as Well as its own, it would be possible for noisecurrents lto build up a bias in the Winding W7 of relay R3 which wouldcause the release or" that relay during the operation of relays R1, R5and Re. This would result in the release of relays R7 and Ra while relayR5 was held operated by the noise currents in L, and the groundconnection over the armature of relay R5 would be broken and theoperation of the suppressor relay R4 prevented in response to the voicecurrents in L'.

Furthermore, it should be noted that noise en ergy passing from east towest in line L' will never cause the operation of relay R1 because thehybrid coil HG2 prevents this energy from entering the detector D1.Accordingly, even though relay R3 is released because of the momentarydecrease of noise, relays R7 and Re will be held operated through thereleased condition ci? relay Re (controlled by relay R1 through relayR5) which maintains the right-hand ground connection for relays R7 andRu.

It remains to point out one other feature of the arrangement shown inFig. 2. If the four-wire circuit shown is connected through the two-wirecircuits to other four-wire circuits, it may be necessary to make theslow release time of relay Re quite longfor instance, .5 second. In sucha case it may be desirable to provide for the canceiiation of the slowrelease period of relay Re whenever the speech energy or the noiseenergy passing from east to west over line L has an eiTect on the relaysat the Westend of the circuit which is stronger than that due to thebiasing current built up by the desensitizing circuits. Thiscancellation may be accomplished with the arrangement shown in thedrawings, whereby an extra armature and an extra contact are added tolrelay R2. When relay R2 operates due to the conditions just discussed,this right-hand contact places a short circuit around the resistance r4'forming a part of the slow release circuit of relay Re.

While the invention has been disclosed in two specific embodiments, thesecond being in the nature of a modification of the rst, it is to beunderstood that such specific disclosure is not intended to limitthescope of the invention, which is determined by the appended claims.

What is claimed is:

l. In a four-wire circuit including paths adapted for transmission inopposite directions, an echo suppressor relatively near each terminal ofthe four-wire circuit adapted to disable the outgoing path in responseto voice currents in the incoming path, means responsivev to noisecurrents for desensitizing said echo suppressor, and means responsive tothe disabling of the incoming path by echo suppressor operation at thedistant end thereof for maintainingon the near echo suppressor thedesensitized effect produced by said desensitizing means.

2. In a four-wire circuit including paths adapted for transmission inopposite directions, echo apparatus comprising means for disabling thesuppressing and associated apparatus relatively near each terminal ofthe four-wire circuit, said outgoing transmission path in response tovoice 'currents in the incoming path, controlling means responsive tovoice currents in the outgoing path for preventing the operation of saiddisabling means by voice currents in the incoming path of later arrival,means responsive to noise currents in either path for desensiti'zingsaid disabling means and said controlling means, and means responsive tothe disabling of the incoming path by echo suppressor` operation at thedistant end thereof for maintaining on said disabling means and saidcontrolling means the desensitized effect produced by said desensitizingmeans.

3. In a four-wire circuit including paths adapted for transmission inopposite directions, the method oi transmission control which consistsin causing voice currents in the incoming path at either end of thecircuit to disable the outgoing path at that end, provided said voicecurrents are or" earlier arrival at that end than voice currents in theoutgoing path at the same end,

causing noise currents in either path to set up a counter action toprevent false interruption of transmission, and maintaining thecounteracting effect at either end of the circuit during a period whentransmission is cut oi at the distant end thereof.

Li. lin a four-Wire circuit including paths adapted for transmission inopposite directions, an echo suppressor relatively near each terminal ofthe four-Wire circuit adapted to disable the outgoing path in responseto voice currents in the incoming path, means responsive to noisecurrents for desensitizing 'said echo suppressor, means responsive tothe disabling of the incoming path by echo suppressor operation at thedistant end thereof for maintaining on the near echo suppressor thedesensitized eiiect produced by said desensitizing means, and meansresponsive to substantial increase of amplitude of the incoming currentsfor terminating the operation of said maintaining 14 means.

SUMNER B. WRIGHT. DOREN MITCHELL.

